When I see the word stimulate, I would not necessarily associate it with either method of regulation that you outline; in other words, I would in general be open to the possibility that either or both might be occurring. In this specific we would need to identify which protein phosphatase this was, in order to answer the question. I believe that protein phosphatase 2A is the enzyme in this case and that xylulose 5-phosphate plays a role in its regulation. I would also like to mention GL protein, which is known to be able to complex to protein phosphatase-1.